! The applicant seeks this K99/R00 award to achieve research independence in intervention science, with a focus on improving emotion regulation and reducing substance use, violent behavior, and sexual risk taking among vulnerable youth. Juvenile offenders (JOs)?who are disproportionately African American?are at high risk for human immunodeficiency virus (HIV)/sexually transmitted infections (STI) due in part to elevations in three interrelated areas underpinned by poor emotion regulation: substance use, violent behavior, and sexual risk taking. Mindfulness-based interventions (MBIs) improve emotion regulation and may thereby reduce HIV/STI risk. However, the effects of MBIs on substance use, violence, and sexual risk taking among JOs are unknown. Importantly, nearly 80% of juveniles are released back into the community on probation following arrest, typically without evidence-based intervention services, positioning them for ongoing risk behavior. Given limited resources within the juvenile justice system, mobile interventions delivered by app are a promising method of reaching these youth. This point is underscored by the fact that African American youth are the racial/ethnic group most likely to own and regularly use smartphones. The applicant thus proposes to adapt an existing app comprised of evidence-based MBI methods in order to specifically address HIV/STI risk behaviors among male and female JOs 13-17 years old on probation (K99 phase), conduct an alpha test of the adapted app (K99 phase), and then run a rigorous test of its efficacy (R00 phase). During the K99 phase, she will collaborate with the makers of the existing MBI app to program modifications to their platform based on focus group feedback (n = 3 groups with n = 5-8 JOs/group), and then address any necessary refinements identified during alpha testing (n = 10 JOs). During the R00 phase, JOs will be randomly assigned to the adapted MBI app (n = 100) or an equally intensive health promotion control app (n = 100). In both conditions, youth will be prompted to complete 10 minutes of guided intervention activity daily for 30 days. Emotion regulation, substance use, violent behavior, and sexual risk taking will be assessed at baseline, post intervention, and six months. The coordinated training plan will allow the applicant to build on her strong foundation in emotion regulation and risk taking in vulnerable youth, developing new skills in three areas critical to her independence: (1) adaptation and design of interventions for high-risk youth, (2) mobile intervention delivery, and (3) efficacy testing. During the K99 Phase, her mentorship team will draw on its sustained track record in mentoring junior scholars to full independence. This will be accomplished through regular meetings, directed readings, hands- on tutorials, and support of her activity in workshops, courses, and conferences. Completion of the R00 phase will generate data to support a future R01 application to test effectiveness and implementation of the adapted MBI on a larger scale. The K99/R00 award will thus provide the applicant with a platform to launch her independent career in intervention science with a focus on emotion and risk behavior in vulnerable youth. !